1. Field of the Invention
The present invention relates to an anemometer of the type comprising a set of ultrasonic emitting and receiving probes arranged such as to define at least three different air paths of propagation of ultrasonic signals, and a device for measuring the ultrasonic signal propagation time along the different paths and for calculating the direction and the speed of the wind based upon the measured propagation times.
2. Description of the Prior Art
Numerous types of anemometers exist, but none provides measurement of wind speed with the desired degree of precision in difficult conditions, notably in freezing or snowy conditions, and with sufficient autonomy.
Moreover, frost significantly perturbs rotating anemometers by modifying the coefficient of friction in mechanical trains. Frost can indeed be eliminated with heating, but this entails unacceptable levels of energy consumption. Furthermore, rotating anemometers may give readings with a high degree of error in extreme conditions (eg. very high wind speed).
Anemometers with no moving parts do not have the drawbacks of rotating anemometers, but their operation is affected by the formation of frost or the settling of snow on their sensitive parts such as the Pitot tube, etc. This sensitivity to frost and snow is also a problem in known ultrasonic anemometers.
The latter have additional drawbacks. In certain cases, the propagation time between the emitting probe and the receiving probe is determined by measuring the time interval between the signal emission and the signal reception, detected by the crossing of a receiving probe output signal threshold. This method proves to be sensitive to parasitic signals and necessitates the emission of relatively high energy signals, reducing the autonomy of the apparatus.
In other cases, the measurement is based on a phase shift between the emitted signal and the received signal. Due to the frequency of the ultrasonic signal and to the distance between the emitting and receiving probes, the phase shift occurs over several periods of the ultrasonic signal. Errors can be thus introduced by loss of the zero phase shift reference.